The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
A spiral wound membrane element is typically made by wrapping one or more membrane leaves and feed spacer sheets around a perforated central tube. The membrane leaves each have a permeate carrier sheet placed between two generally rectangular membrane sheets. The membrane sheets are typically sealed together along three edges. The fourth edge of the leaf is adjacent to the central tube and open to the perforations. One or more layers of permeate carrier sheet can also be wrapped around the central tube to support the membrane leaf over the perforations in the central tube and to provide a flow path between the edge of the leaf and the central tube. An anti-telescoping device (ATD) is attached at the ends of the membrane element to prevent telescopic unraveling of the membrane element.
Feedstock, also referred to as feed water, is introduced at one end of the membrane element and flows through the feed spacer sheets and along the surface of the membrane sheets. Some of the feedstock passes through the membrane sheets to form a permeate stream. The remainder of the feedstock, referred to as the reject, retentate or brine stream, flows through the feed spacer sheets and out of the outlet end of the membrane element. The permeate stream flows along the permeate carrier in a inwardly spiraling flow. The permeate stream follows the permeate carrier until reaching and exiting the fourth edge of the leaf and entering the central tube. Within the central tube, the permeate stream is collected and transported towards an outlet end of the central tube.
The throughput or collection rate of permeate in a spiral wound membrane is related to the pressure applied across the membrane. However, the pressure required to drive the permeate flow through the permeate carrier, including from the edges of the permeate carrier, towards the central tube reduces the net driving pressure for permeate flow through the membrane.